1. Field of the Invention
The present invention relates to a casting method and a casting apparatus, and more particularly to a casting method and a casting apparatus in which a cast product having a desired shape is cast by allowing molten metal poured into an cavity of a molding die and a reducing compound to be contacted with each other whereby an oxide film formed on a surface of the above-described molten metal is reduced.
2. Description of the Related Art
There exist various types of aluminum casting methods such as, for example, a modified aluminum casting method proposed in Japanese Patent Application No. 108078/2000 by two inventors of the present application.
A molding die to be adopted by this modified aluminum casting method is shown in FIG. 8. The molding die 100 thus shown in FIG. 8 is such a molding die made of metal as is used in a gravity casting method; on this occasion, the molding die is of a separate type comprising a lower die 102a and an upper die 102b. By these dies 102a and 102b, formed is a cavity 104 in which a cast product having a desired shape is cast.
Further, in the upper die 102b, a feeder head portion 108 is formed between a sprue 106 from which molten metal of aluminum or an alloy thereof is poured and the cavity 104, and also air-vent holes 110 from which an air in the cavity 104 is discharged when the molten metal is poured into the cavity 104 is formed.
In the improved aluminum casting method using such molding die 100, after a reducing compound, that is, a magnesium-nitrogen compound (Mg3N2) is introduced into the cavity 104 of the molding die 100, the molten metal of aluminum or the alloy thereof is poured into the sprue 106 of the molding die 100 and, then, the molten metal is filled in the cavity 104 and the feeder head portion 108 while the air is discharged from the air-vent holes 110.
Next, the molten metal in the cavity 104 is solidified by cooling the molding die 100 in which the molten metal is filled in the cavity 104 and the like as it stands still. A void which is caused by shrinkage with solidification of the molten metal is supplemented by allowing a part of the molten metal in the feeder head portion 108 to be flowed down in the cavity 104.
The improved aluminum casting method is a reduction casting method in which an oxide film formed on a surface of the molten metal of aluminum or the alloy thereof is reduced in the presence of a reducing compound within the cavity 104 of the molding die 100 to decrease a surface tension of the molten metal and, as a result, a flowing property and a running property of the molten metal can be enhanced.
For this feature, in the improved aluminum casting method, coating of a coating agent which is to be coated on surfaces of inner walls of the feeder head portion and the cavity aiming for enhancement of a flowing property and the like of the molten metal and the like on which the oxide film is formed can be omitted thereby enabling to promote a reduction of production steps and enhance a transferring property of the molding die 100.
Now, depending on the shapes of the cast products, there is a case in which the cavity 104 of the molding die 100 is forced to have a shape where a narrow portion having a smaller cross-sectional area than that of a terminal portion is formed halfway between the sprue and the terminal portion. For example, there is a case in which the cavity 104 is forced to have a shape where a first cavity portion 104a in which a molten metal inlet of the cavity 104 is arranged and a second cavity portion 104b, that is, the terminal portion are connected with a narrow portion 104c which is formed narrower than the first cavity portion 104a and the second cavity portion 104b (hereinafter, also referred to only as cavity portion 104a and cavity portion 104b respectively, or as cavity portions 104a and 104b collectively).
In the cavity 104 shown in FIG. 9, after the reducing compound, that is, the magnesium-nitrogen compound (Mg3N2), is introduced into the cavity 104 of the molding die 100, the molten metal of aluminum or the alloy thereof poured into the sprue 106 is then poured into the first cavity portion 104a and, thereafter, poured into the second cavity portion 104b via the narrow portion 104c. Such pouring, i.e., filling of the molten metal in the cavity 104 is performed in a short period of time by allowing an oxide film formed on the surface of the molten metal to be reduced in the presence of the reducing compound.
However, since the molten metal filled in the narrow portion 104c of the cavity 104 is smaller in quantity than that in the cavity portions 104a and 104b and faster in cooling rate than that filled in the cavity portions 104a and 104b, the molten metal filled in the narrow portion 104c is solidified earlier than that filled in the second cavity portion 104b. 
For this reason, even when the void is formed while shrinkage is generated with the solidification of the molten metal filled in the second cavity portion 104b, the second cavity portion 104b is not replenished with the molten metal filled in the first cavity portion 104a and the feeder head portion 108, that is, an effect of feeding the molten metal can not be expected whereupon there is a fear that a shrinkage hole or the like may be generated in an obtained cast product.
Meanwhile, though it is possible to solve the shrinkage hole or the like to be generated with the solidification of the molten metal filled in the second cavity portion 104b by independently arranging the feeder head portion in each of the cavity portions 104a and 104b, such an arrangement as forms feeder head portions in a plurality of different places will lead to a complexity of a constitution of the molding die.
Further, since a part of the molten metal which is solidified in the feeder head portion 108 is not a cast product, the portion is cut off to be disposed. Even when it is considered that the thus-cut off portion is reused after being melted again, a loss of energy must be expected.
Therefore, forming feeder head portions in a plurality of different places increases a capacity of a part of non-cast product, decreases a yield of the cast product of the molten metal poured into the molding die 100 and, accordingly, increases a loss in workability and energy.